In order to control the attitude of a spacecraft, various rotating inertia members such as reaction wheel assemblies (RWAs), control momentum gyros (CMGs) and similar actuators are used.
Reaction wheels are typically flywheels that are fixed in the body frame. Torque can be produced by varying the spin rate of the flywheel. The torque produced is produced along the spin axis of the flywheel. A CMG comprises a flywheel with a fixed spin rate mounted to a gimbal assembly. The spin axis of the CMG can be changed by moving the CMG using the gimbal assembly. The torque produced is orthogonal to the spin axis and the gimbal axis. Typically, multiple actuators are oriented such that manipulation of each actuator causes movement about one of at most three linearly independent axes. Thus, control of all three attitude degrees of freedom requires at least three actuators, with extra actuators added to provide for failure robustness, to distribute control authority among the various attitude degrees of freedom, and to enable subtle steering laws. It is common practice to distribute the actuators remotely from each other, placing them wherever space permits. While distributing the actuators results in a system that generally applies the correct control torques, inefficiencies can result, including duplication of actuator electronics, mechanical structure, and vibration-isolation hardware among these remotely placed units.
To solve these problems, various momentum control systems (MCS) have been proposed. In one embodiment, the MCS comprises at least three actuators mounted together on a single mechanical pallet. A number of isolation struts, comprising the mechanical equivalent of one or two springs and a viscous damper, are coupled at one end to the pallet and at another end to the spacecraft (via, in most cases, a pivot with low rotational stiffness but high translational stiffness, such as a ball-in-socket joint or a blade flexure). The isolation system acts as a mechanical low-pass filter that transmits torque produced by manipulation of the actuators to the spacecraft while at the same time attenuating higher-frequency vibrations. Such an arrangement can be found in U.S. Pat. No. 6,340,137 entitled “Moment Control Unit for Spacecraft Attitude Control” and issued to Davis et al. on Jan. 22, 2002. This patent is hereby incorporated by reference.
Isolation struts can be either passive struts or active struts. Typical passive struts utilize a viscous fluid to help attenuate high-frequency vibrations. An exemplary passive strut is described in U.S. Pat. No. 5,249,783 issued to Davis on Oct. 5, 1993. This patent is hereby incorporated by reference.
Active struts are similar to passive struts but also include sensors and a prismatic actuator in parallel or in series with the passive elements. The actuator is typically a Lorenz force actuator, such as a voice coil, although any actuator can be used. Sensors are used to detect displacements, accelerations, loads or other parameters. The actuator, in response to the sensors or to external commands, applies forces to filter out or actively suppress these measured disturbances. Another actuator can be used to actively change the fluid pressure in the strut to enhance the vibration and shock dissipation. An exemplary active strut is described in U.S. Pat. No. 6,003,849 issued to Davis et al. on Dec. 21, 1999. This patent is hereby incorporated by reference.
In current momentum control systems (MCSs), the use of active struts is limited to filtering disturbances and to responding to the movement of the platform as caused by the movement of the CMGs or RWA. However, the struts can be used for more than merely reacting to these loads. Therefore, what is needed is a method and system for steering a momentum control system that takes full advantage of the functionality inherent in the active-strut design.